1. Field of the Invention
The present invention relates to a device for applying a foamed hot melt adhesive, and more specifically, the present invention relates to a device for applying the foamed hot melt adhesive for applying a foamed hot melt adhesive in which air bubbles are mixed by mixing gas in a solution of the hot melt adhesive.
2. Description of the Related Art
With respect to this kind of a device for applying a foamed hot melt adhesive having air bubbles mixed therein and a method for selectively applying a foamed hot melt adhesive having air bubbles mixed therein, JP-B-60-3350 (Japanese Patent Application No. 52-91500, Japanese Patent No. 1380841, “device for manufacturing hot melt heating plasticity adhesive foam”) is publicly known.
The publicly-known art according to the above-described JP-B-60-3350 includes a hot melt adhesive compression process using a first gear pump 101 with reference to FIG. 4 and a gas mixing process using a second gear pump 102 and discloses the art to discharge a gas mixed hot melt adhesive from a front end of a nozzle 103 and make the gas mixed hot melt adhesive into a foam condition under instantaneously an atmosphere pressure.
According to the above-described conventional art, the first gear pump and the second gear pump are driven by a single drive mechanism to make the both gear pumps into the same capacity so as to generate a negative pressure in a pressure-feed process of two stages and mix gas in the hot melt adhesive.
In this case, a gear ratio is constant, so that it is necessary to always regulate a gas supplied quantity in order to gear the control of increase and decrease of the mixed air bubbles to the supplied quantity of the consumed hot melt adhesive. In other words, if the ratio of a ratio of a supply capacity of the both pumps is made larger (namely, the negative pressure is made larger), the air bubbles become too big and this involves a problem such that the bubbles may burst or may be crushed by the atmosphere pressure after they are discharged. On the contrary, if the ratio is made smaller, this involves a problem such that it takes a long tome to mix the air bubbles.
A right capacity ratio of the pumps is about 1:1.5, however, according to the above-described conventional art, the first gear pump and the second gear pump are integrated and are driven on the same axis, so that it is not possible to control the value of the forced negative pressure by rotation frequencies of the pumps. The inlet quantity of the supplied gas is limited. In order to maintain the mixing rate of the gas immediately in line with the discharge quantity of the gas, it takes a long time. Further, this involves a problem such that the condition for making the optimum and fine air bubbles quickly cannot be obtained.